1. Field of the Invention
The present invention relates to a method and apparatus for diagnosing leakage of a fuel vapor treatment unit of an internal combustion engine, and in particular to technology for preventing erroneous diagnosis.
2. Description of the Related Art
With conventional fuel vapor treatment units for internal combustion engines, evaporation of fuel vapor into the atmosphere is prevented by temporarily adsorbing fuel vapor produced in the fuel tank etc. into a canister, and then, under predetermined engine operating conditions, de-adsorbing the adsorbed fuel vapor and mixing this with purge air, and drawing the purge mixture into the engine intake system, while controlling the flow with a purge control valve (refer to Japanese Unexamined Patent Publication No. 5-215020).
With the above units however, if a crack occurs along the fuel vapor piping, or a fault occurs in a seal at a fuel vapor piping connection, then the fuel vapor will evaporate into the atmosphere from the leak portion, so that the original evaporation prevention effect cannot be fully realized.
For an apparatus for diagnosing the presence of such fuel vapor leaks, the following methods have been contemplated.
That is to say, there is an apparatus which diagnosis the presence of fuel vapor leaks by setting a judgment level based on a drive current of an electric pump for when air is pumped by the electric pump via a reference orifice having a reference aperture diameter, and then comparing a drive current of the electric pump for when air is pumped by the electric pump bypassing the reference orifice and into piping to be leak diagnosed of the fuel vapor treatment unit, with the set judgment level. More specifically, when the drive current is less than the judgment level, it is diagnosed that a fuel vapor leak has occurred. That is to say, when a leakage amount is produced which is larger than the leakage amount for when a hole equivalent to the reference orifice occurs, then due to the reduction in air pumping load the drive current of the electric pump falls below the judgment level. Hence the presence of leaks can be diagnosed by comparison with the judgment level.
With the above method, it is possible to diagnose to a high accuracy, even when a small leakage amount is produced such as in the case where a very small hole occurs in the piping.
However, with the above method, if due to the occurrence of blockage or contamination in the reference orifice, the drive current of the electric pump for when air is pumped through the reference orifice increases markedly and the presence of leaks is diagnosed using a judgement level set based on this drive current, there is the possibility of an erroneous diagnosis of the presence of a leak in the case where there is actually no leak.
Furthermore, with the method for performing leakage diagnosis as described above after stopping operation of the engine, if leakage diagnosis is executed at the time of opening the filler cap of the fuel tank for refuelling, since the interior of the fuel vapor supply system is opened to the atmosphere, then the drive current of the electric pump will be reduced so that there will be an erroneous diagnosis that a leak has occurred. This erroneous diagnosis situation is not limited to pressurising methods using an electric pump. For example a similar erroneous diagnosis can also occur with a method where pressure stored inside an accumulator during engine operation is supplied to inside the fuel vapor supply system, and diagnosis is made based on a subsequent pressure change.